Dual connectivity has been one of the recent features in LTE-Advanced. 5G networks are expected to use dual connectivity (DC) and multi-connectivity (MC) for supporting services with enhanced coverage, reliability and/or throughout requirements. Multi-connectivity can be used for both extreme mobile broadband and machine type communications (MTC), for example throughput and reliability for mobile broadband video, and ultra-reliable communications for MTC. Multi-connectivity may further leverage the additional spectrum that will be available for 5G networks. Thus for example, the master node may act as the anchor carrier in one 5G frequency band while the secondary cells may be configured to transmit in other frequency bands.
Packet delivery of certain services and applications have certain reliability requirements and certain latency targets. For example, a video content is served to a user with a desirable latency guarantee without dropping too many packets and adversely affecting the user experience. Other examples involve industrial MTC (industrial automation) scenarios that may also require stringent latency requirements with minimal packet drop targets.
Multi-connectivity is one way of supporting services in future 5G networks, wherein multi-node carrier aggregation can be employed to use multiple frequency bands. A particular type of multi-connectivity may split a bearer or an Internet Protocol (IP) flow between multiple transmitting nodes such as primary and secondary base stations which may not be geographically co-located. In such a multi-connectivity scenario (sometimes known as dual-connectivity DC or multi-connectivity MC) data is transmitted via two or more base stations such that a portion of the data is transmitted via a master base station and a different portion of the data is sent via one or more secondary base stations. The data split is done at the packet data convergence protocol (PDCP) layer, with some of the PDCP packets sent by the master base station and some of the packets sent by the secondary base station. The user equipment receives the data from the multiple nodes (base stations), and re-orders the PDCP packets at the PDCP layer.
In practical deployments poor link quality can result in dropped packets from either the master or secondary base stations which could impact the reliability of packet delivery. For instance, the master node could be facing a severe congestion with many users to be served. In that case some of the packets that are to be served to a far-away UE that is experiencing poor link quality may be dropped from the transmission buffer of the master node. In some implementations, the packets may be transmitted but incorrectly received by the UE. If the master node resolved this by simply adjusting the split bearer ratio so as to assign more of this UE's packets to the secondary node), the secondary node's radio link could become overloaded because the frequencies configured for the master node may not be utilized for data. This may result in a situation that the radio layer of the secondary node which may be configured with higher frequencies would not be able to meet the required throughput for the service when it tries to provide increased reliability. The problem then is to achieve a good trade-off between throughput and ultra-reliability. As detailed below, embodiments of these teachings can increase both the achieved throughput and the achieved reliability of packet reception at the UE via a dynamic mechanism of higher layer re-transmissions; this is well adapted for latency critical traffic in 5G systems. Packet re-transmission is well known in the wireless arts as re-sending a packet in response to a negative acknowledgement (NACK) from the intended recipient. In practice a NACK may be indicated by the absence of an acknowledgement (ACK). Transmitted packets are tracked by the respective sender in hybrid automatic repeat request (HARQ) processes which define the exact radio resources on which to send the re-transmission.